An example of radiographic apparatus is an imaging apparatus that obtains X-ray images by detecting X rays. This apparatus used an image intensifier as an X-ray detecting device in the past. In recent years, a flat panel X-ray detector (hereinafter called simply “FPD”) has come to be used instead.
The FPD has a sensitive film laminated on a substrate, detects radiation incident on the sensitive film, converts the detected radiation into electric charges, and stores the electric charges in capacitors arranged in a two-dimensional array. The electric charges are read by turning on switching elements, and are transmitted as radiation detection signals to an image processor. The image processor obtains an image having pixels based on the radiation detection signals.
The FPD is lightweight and free from complicated detecting distortions compared with the image intensifier used heretofore. Thus, the, FPD has advantages in terms of apparatus construction and image processing.
However, when the FPD is used, the X-ray detection signals include lag-behind parts. A lag-behind part results in an afterimage from X-ray irradiation in a preceding imaging event appearing as an artifact on a next X-ray image. Particularly, in a fluoroscopy that performs X-ray irradiation continually at short time intervals (e.g. 1/30 second), time lags of the lag-behind parts have influences serious enough to hinder diagnosis.
Artifacts due to lag-behind parts are reduced by reducing long time constant components of the lag-behind parts by using backlight (see Patent Document 1, for example), or by regarding the lag-behind parts as a total of exponential functions having a plurality of time constants, and performing a lag correction by recursive computation using these exponential functions (see Patent Document 2, for example).
Where backlight is used as disclosed in the Patent Document 1 noted above, the construction becomes complicated by a construction required for backlight. Particularly where backlight is used in an FPD having a lightweight construction, the construction must become heavy and complicated again. In the case of Patent Document 2, the lag correction must be carried out by performing recursive computations the number of times X-ray detection signals are sampled. This renders the lag correction complicated and cumbersome.
In order to remove lag-behind parts included in X-ray detection, signals simply from the X ray detection signals, it is conceivable in performing a lag correction, to acquire a plurality of X-ray detection signals in time of non-irradiation before irradiation of X rays in an imaging event, acquire a lag image based on the X-ray detection signals, and using this image to remove the lags from a product X-ray image.
In addition to lag correction, correction processes include offset correction, gain correction, and defect correction, for example. To perform offset correction, for example, an offset image is obtained beforehand in time of non-irradiation. The above offset image is subtracted from an original image based on X-ray detection signals. An offset image is different for each mode such as storage time, amplification, factor (gain) of an amplifier or pixel binning (addition of a plurality of pixels). An offset image according to a mode is obtained to perform offset correction, (see Patent Document 3, for example). Pixel binning includes a 1×1 mode which outputs pixels in 1 to 1, a 2×2 mode which outputs four pixels of 2×2 in rows and columns to one pixel, and a 4×4 mode which outputs 16 pixels of 4×4 in rows and columns to one pixel.
[Patent Document 1]
Unexamined Patent Publication No. H9-9153 (pages 3-8, FIG. 1)
[Patent Document 2]
Unexamined Patent Publication No. 2004-242741 (pages 4-11, FIGS. 1 and 3-6)
[Patent Document 3]
Unexamined Patent Publication No. 2003-190126 (pages 3-6, FIG. 1)